In the course of assessing and producing hydrocarbon bearing formations and reservoirs, it is important to acquire knowledge of formation and formation fluid properties which influence the productivity and yield from the drilled formation. Typically such knowledge is acquired by methods generally referred to as “logging”.
Logging operations involve the measurement of a formation parameter or formation fluid parameter as function of location, or more specifically depth in a wellbore. Formation logging has evolved to include many different types of measurements including measurements based on acoustic, electro-magnetic or resistivity, and nuclear interactions, such as nuclear magnetic resonance (NMR) or neutron capture.
NMR measurements are commonly used in the wellbore to probe the NMR decay behavior of the stationary fluid in the reservoir rock. During these measurements, magnetic fields are established in the formation using suitably arranged magnets. The magnetic fields induce nuclear magnetization, which is flipped or otherwise manipulated with on-resonance radio frequency (RF) pulses. NMR echoes are observed, and their dependence on pulse parameters and on time is used to extract information about the formation and the fluids in it.
In particular, NMR has been used in the oilfield industry to obtain information and parameters representative of bound fluids, free fluids, permeability, oil viscosity, gas-to-oil ratio, oil saturation and water saturations. All these parameters can be derived from measurements of spin-spin relaxation time, often referred to as T2, spin-lattice relaxation time (T1), and self-diffusion coefficient (D) of the molecules containing hydrogen contained in formation fluids.
On the other hand, fluids are routinely sampled in the well bore with the help of so-called formation testers or formation fluid sampling devices. An example of this class of tools is Schlumberger's MDT™, a modular dynamic fluid testing tool. Such a tool may include at least one fluid sample bottle, a pump to extract the fluid from the formation or inject fluid into the formation, and a contact pad with a conduit to engage the wall of the borehole. When the device is positioned at a region of interest, the pad is pressed against the borehole wall, making a tight seal and the pumping operation begins.
When specifically attempting to inject rather than extract fluid from the formation, a testing tool may require modifications such as described for example in the co-owned U.S. Patent Application Publication No. 2006/0000606. The tool described therein is a formation tester for open hole formations incorporating a drill bit to drill through the mudcake which accumulates on the wall of the well bore or through zones damaged or contaminated by the drilling process. The tool as described in U.S. Publication No. 2006/0000606 is capable of injecting fluid into the formation surrounding wellbore for various purposes such as fracturing the formation near the wellbore.
It is further well established to mount logging tools on either dedicated conveyance means such as wireline cables or coiled tubing (CT) or, alternatively, on a drill string which carries a drill bit at its lower end. The latter case is known in the industry as measurement-while-drilling (MWD) or logging-while-drilling (LWD). In MWD and LWD operations, the parameter of interest is measured by instruments typically mounted close behind the bit or the bottom-hole assembly (BHA). Both logging in general and LWD are methods known as such for several decades and hence are believed to require no further introduction.
Also known for as many as three decades are measurements broadly referred to as log-inject-log or LIL measurements. From their inception as exemplified by U.S. Pat. No. 3,562,523 issued to Richardson and Wyman, LIL based methods have evolved into many variants.
However, the basic principle of LIL can still be seen as including a sequence of steps starting with the use of a first drilling fluid system, which invades the formation at a first time and then logging the invaded formation for the desired parameter. In a subsequent step, the composition of the drilling fluid is changed and this new fluid is circulated. After the new fluid has invaded the rock formation, the logging operation is repeated. The second measurement thus registers the change in the value or spatial distribution of the parameter in question.
Known variants of the LIL methods are described for example in U.S. Pat. No. 3,748,474 to Murphy, U.S. Pat. No. RE 28,963 to Fertl and Reynolds and in U.S. Pat. No. 4,987,368 to Vinegar. Further reference to LIL can be found in U.S. Pat. No. 5,796,252 to Kleinberg et. al. The latter document provides also further insight into and details of state-of-the-art NMR logging.
In a paper prepared for presentation at the SPWLA 1st Annual Middle East Regional Symposium, Apr. 15-19, 2007, Gilles Cassou, Xavier Poirier-Coutansais and one of the inventors of the present invention, Raghu Ramamoorthy, demonstrate that the combination of advanced-NMR fluid typing techniques with a dual-packer fluid pumping module can greatly improve the estimation of the saturation parameter in carbonate rocks. The ability to perform 3D-NMR stations immediately before and after pump-outs yields both the water and oil saturations (Sw,Sxo) independently of lithology, resistivity, and salinity in a complex carbonate environment.
In view of the known art, it is therefore seen as one object of the invention to improve and enhance known methods for characterizing formations using fluid injection into the formation. It is seen as another object to provide more and better methods of determining characteristic formation and formation fluid properties to monitor the performance of fluid for the purpose of increasing hydrocarbon production from these formations.